In different parts of electrical transformers, insulating material is used to avoid flash-overs and such. This insulating material is typically cellulose based since such a paper or pressboard material is cheap and easy to handle while giving good insulation and has suitable mechanical, electrical and thermal properties. Examples of insulators in an oil filled transformer are:                spacers, positioned between the turns/discs of a winding, allowing oil to circulate there between.        axial sticks, positioned between the winding and the core, or between different windings.        cylinders positioned around a winding, between the winding and its core, or between different windings.        winding tables, positioned atop and below the plurality of windings, supporting the same.        insulation coating of the conductor of the windings.        
Pressboard is a class of cellulose-based material, typically constructed of one or several layers (plies) of paper which, when compressed using a combination of heat and pressure, form a stiff, dense material in a range of weights.
Pressboard has been used as insulation material in power transformers for many years. The composition and manufacturing process of pressboard have remained basically unchanged for as many years. There are a number of reasons for such lack of innovation. Pressboard mainly offers, at a relatively cheap price, good mechanical and electrical properties. In addition to that, easy machinability and versatility in the workshop increase the value of the material.
However, there are some aspects of the pressboard material that could desirably be improved. These aspects are mainly related to mechanical properties of the material. A challenge is to improve the in-plane and out-of-plane mechanical properties of pressboard, without degrading the dielectric properties thereof. Improved in-plane stiffness and strength would bring about higher bending stiffness of both single sheet and laminate materials. Higher rigidity in the out-of-plane helps both during manufacturing process and during transformer life time.
It is important to bear in mind that in-plane and out-of-plane properties are not directly connected, in the sense that an improvement of the first does not necessarily cause an improvement of the second and vice versa.
U.S. Pat. No. 6,736,933 discloses a multi-ply paperboard comprising at least one ply of conventional cellulose fibers and from about 0.1 to about 6 weight percent of a water-borne binding agent, and at least one ply of chemically intrafiber crosslinked cellulosic high-bulk fibers and from about 0.1 to about 6 weight percent of a water-borne binding agent. The water-borne binding agent may be a starch, a modified starch, a polyvinyl alcohol, a polyvinyl acetate, a polyethylene/acrylic acid copolymer, an acrylic acid polymer, a polyacrylate, a polyacrylamide, a polyamine, guar gum, an oxidized polyethylene, a polyvinyl chloride, a polyvinyl chloride/acrylic acid copolymer, an acrylonitrile/butadiene/styrene copolymer or polyacrylonitrile.